Avoid negative space in \bmod. (#984)

This is a workaround for #836 for the command case of using \bmod,
without changing any spacing (and making the code and generated output
slightly more efficient).  In particular, fix #982.

static/katex.less

@@ -118,6 +118,7 @@
     @ptperem: 10.0;
     @nulldelimiterspace: 1.2em / @ptperem;
 
+    @muspace: 0.055556em;       // 1mu
     @thinspace: 0.16667em;      // 3mu
     @mediumspace: 0.22222em;    // 4mu
     @thickspace: 0.27778em;     // 5mu
@@ -327,6 +328,10 @@
             margin-left: -@thinspace;
         }
 
+        &.muspace {
+            width: @muspace;
+        }
+
         &.thinspace {
             width: @thinspace;
         }